(i) Field of the Invention
This invention relates to melanin inhibitors and more particularly melanin inhibitors comprising a specific type of a cinnamic acid derivative as the effective component thereof.
(ii) Description of the Prior Art
Freckles, chloasma and pigmentary deposits after sunburn tend to occur or increase or become difficult to disappear with increasing age, thus being one of serious problems on skin care to persons of the middle or advanced age. The critical mechanism of these pigmentations is not known at present but it is considered that melanogenesis of melanocyte is facilitated by the action of ultraviolet rays and melanocyte-stimulating hormones. Moreover, it is considered that the tendency toward the retardation of keratinization of keratinocytes cells accompanied by the increase of age will retard the discharge of melanin to outside of the epidermis, thus developing, along with the acceleration of melanogenesis, symptoms of increasing the density of melanin granules in the epidermis or clinically increasing the pigmentary deposits.
Now, there is a high demand of medicines which act to restore acquired deposit of a pigment i.e. melanin to the normal skin color. A number of medicines have been up to now developed and put into practice. For instance, peroxides are believed to bleach melanin and thus attempts have been made in the use of hydrogen peroxide, zinc peroxide, sodium peroxide, benzoyl peroxide and the like. However, these peroxides are unstable compounds and little effects of reducing the pigmentation were not observed in practical application conditions. In recent years, cosmetics which comprise vitamin C (L-ascorbic acid) having good reducing ability were proposed, however, it showed little effects in external applications. Moreover, vitamin C is rather unstable and have disadvantage to be comprised in cosmetics.
On the other hand, in Europe and the United States of America, hydroquinone and derivatives thereof, various catechols have been used for treatment of moth patches or bleaching of colored man's skin. However, these compounds have safety problems (such as high stimulative, allergic troubles and the like) and may sometimes cause white spots, thus compounding of these substances as the medicine being rather disadvantageous. Use of a variety of melanin inhibitors has been also reported but almost all the substances showed little melanin inhibiting effects.